PS One of my uppers was made by Amercan Spirit Arms. It is rather neat, with a charging handle attached to the bolt on the left side of the upper. This allows you to work the bolt while the gun is still on your shoulder. After I had assembled the upper and tried to attach it to the lower, I had a problem; it wouldn't close. Closer inspection revealed that the MFR had left a "lump" in the area where the upper and lower came together, just behind the forward pin. Careful removal of metal from the "lump" allowed the upper and lower to close and provided a nice tight, no wobble fit. Too bad they went out of business.